Categorization of cold period weather types in Greece on the basis of the photointerpretation of NOAA/AVHRR imagery

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1 INT. J. REMOTE SENSING, 10 AUGUST, 2004, VOL. 25, NO. 15, Categorization of cold period weather types in Greece on the basis of the photointerpretation of NOAA/AVHRR imagery C. CARTALIS University of Athens, Department of Applied Physics, Building PHYS-V, Athens, GR Greece N. CHRYSOULAKIS* Foundation for Research and Technology Hellas, Institute of Applied and Computational Mathematics, Regional Analysis Division, P.O. Box 1527, Heraklion, Crete, GR Greece; H. FEIDAS University of the Aegean, Department of Geography, University Hill, Building of Geography, GR Mytilene, Greece and N. PITSITAKIS National Meteorological Service, Hellinikon, Athens, Greece (Received 9 July 2002; in final form 11 July 2003 ) Abstract. The Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) satellites may be used to detect and monitor prevailing synoptic weather systems and to describe the state of the atmosphere with very good spatial and temporal resolution. In this study, the synoptic weather types which are responsible for severe cold and rainy weather conditions in Greece during the cold period of the year were examined with the use of NOAA/AVHRR satellite images in conjunction with weather charts. An attempt was made to develop a classification scheme for these synoptic weather types on the basis of their tracks and cloud patterns inferred by satellite images. Forty-two visible and thermal infrared AVHRR images were interpreted with emphasis on the analysis of cloud patterns formed in synoptic weather systems in order to relate cloud features in every image to physical processes. Synoptic maps (12:00 UTC) from the European Centre for Medium-Range Weather Forecasts (ECMWF) were used as auxiliary data. The final product was a categorization of the cloud features formed by each of the four prevailing weather types in the area of Greece for the winter rainy period (three Depressional Weather Types and one Mixed Weather Type). 1. Introduction The Mediterranean basin is well known as a region of frequent cyclone formation and is affected by moving depressions generated either in the Atlantic *Corresponding author. International Journal of Remote Sensing ISSN print/issn online # 2004 Taylor & Francis Ltd DOI: /

2 2952 C. Cartalis et al. Ocean or in north-western Europe. Preferred regions for cyclogenesis in the Mediterranean region were identified by Radinovic (1987). The depressions occurring in specific areas of the Mediterranean region and the cyclonic tracks have been the subject of extensive climatological research (e.g. Maheras 1979, 1983, 1988a, Katsoulis 1980, Prezerakos 1985, Flocas 1988, Kassomenos et al. 1998). In these studies, the depressions were identified and classified manually on the basis of synoptic charts. An automatic classification of circulation types in Greece has been recently developed using spatial methods of topology and geometry by Maheras et al. (2000, 2001). The central Mediterranean is affected by depressions in the westerly circulation and at the same time is largely influenced by meridional circulations and depressions formed over the western and central Mediterranean or over the Sahara Desert. The meridional circulation is the main factor governing most of the precipitation over the whole of the Mediterranean basin (Maheras 1988a, b, Maheras et al. 1992). The cold period in Greece occurs between late October and early April. The duration of the cold period is of significant importance to the country, because during this period the largest amount of rain is accumulated. The main synoptic weather types which affect the broader area of Greece in winter are classified by Maheras (1979, 1983, 1988a) as follows:. Anticyclonic Weather Type, in which an anticyclone is positioned peripherally to or over Greece. This weather type favours fair and dry weather;. Depressional Weather Type, in which a depression follows a zonal track from west to east or a semi-meridional track from north-west to south-east. This weather type favours cold and rainy weather with strong winds; and. Mixed Weather Type, in which an anticyclone is combined with a depression causing a tied pressure gradient over Greece. Mixed Weather Types cause severe cold weather, with rainfall and snowfall mainly over central and northern Greece, with very low temperatures and very strong north-easterly winds. Despite the considerable research on the climatology of Mediterranean cyclones, there is still a need to correlate depressional weather types with cloud types and patterns developed over an area with various terrain features like Greece. In this study, weather types which are responsible for severe cold and rainy weather conditions in Greece during the cold period are identified on the basis of their tracks and cloud patterns inferred by satellite images in conjunction with weather charts. Afterwards, the interpretation of a large number of related satellite images was carried out, in order to categorize the cloud features in the area of Greece formed by each prevailing weather type. 2. Data and methodology For the purposes of this study, high spatial resolution Advanced Very High Resolution Radiometer (AVHRR) images (Local Area Coverage) were used as acquired from the National Oceanic and Atmospheric Administration (NOAA) ground receiving station of the National Meteorological Service. AVHRR has a spatial resolution of 1.1 km at the nadir and a swath coverage of 2700 km. AVHRR records incoming radiation using five spectral channels: channel 1, mm (visible); channel 2, mm (near-infrared); channel 3, mm (midinfrared); channel 4, mm (thermal infrared) and channel 5, mm

3 Categorization of weather types using satellite imagery 2953 (thermal infrared). In this study, only channels 1 and 4 were used for the photointerpretation. The spatial resolution of AVHRR data (1.1 km) enables the identification of cloud patterns linked with small-scale physical processes and permits a more detailed cloud analysis in comparison to the coarse spatial resolution of the geostationary data ( in the area of Greece). Forty-two cases were selected for days when well-developed low-pressure systems were located over the broader area of Greece (table 1). Fifty-nine AVHRR images (one to two images per day) were used, depending on the passage of NOAA- 14, NOAA-12 and NOAA-9 over the area of east Mediterranean, during the periods January to April and October to December At least one image per day was daytime (reception times close to 12:00 UTC), therefore the best distinctness was achieved for the visible channel. Synoptic analysis maps (12:00 UTC) from the European Centre for Medium- Range Weather Forecasts (ECMWF) have also been used. These maps presented graphically the horizontal distribution (over Europe and the Mediterranean Sea) of the following parameters:. 500 hpa contour lines;. 500 hpa isotherms;. 500 hpa isotachs of vertical velocity;. surface isobars; and. 850 hpa isotherms. Synoptic analysis maps were used as auxiliary data for the photointerpretation. The analysis of cloud patterns was used to estimate the synoptic system types as well as their location. The type and the location of the synoptic systems were verified using the respective synoptic analysis maps. The analysis of cloud patterns on satellite images was also related to physical processes. This relation was also verified using the synoptic analysis maps, e.g. the detection of cumulonimbus clouds on the satellite image was verified using the 500 hpa isotachs map of vertical motion in which convective motion areas can be located. The combination of visible with thermal infrared imagery photointerpretation was necessary to define the atmospheric mechanisms. Clouds and cloud system identification on satellite images were based on shape, texture and pattern recognition relating to the spatial arrangement of cloud elements, on indications of height revealed directly in the infrared imagery and deducible in visible imagery, and the relation to local topography. Clouds viewed in satellite imagery were classified into three general categories based on appearance: cumuliform, stratiform and cirriform. The characteristics of the individual cloud types which form the cumuliform, stratiform and cirriform categories are summarized in tables 2, 3 and 4 (Rao et al. 1990). Detection of fronts has been accomplished using the basic principles of identification of weather systems on satellite imagery (Rao et al. 1990, Bader et al. 1995). Briefly, a classical cold frontal cloud band is defined as continuous, relatively broad cloud formation with mostly cold tops, bright in the visible images, characterized by a distinct long axis with or without curvature. It has a sharp rear edge and sometimes a distinct forward edge where the surface front is located. The wide zone of overcast cloudiness is on the cold side of the surface frontal position. Concerning warm frontal bands, if there is little or no high cloud in the warm sector, the warm frontal position may be well defined by a band of cold clouds in the infrared image. However, high cloud in the warm sector may merge with the

4 2954 C. Cartalis et al. Table 1. Date and time of AVHRR images used for the categorization of the cloud features in the area of Greece for the 42 cases examined in this study. The corresponding synoptic weather type for each case is also included. Day Time (UTC) Weather type 1 4 January :20, 19:45 WDWT 2 9 January :57 WDWT 3 12 January :31, 19:52 WDWT 4 14 January :53 SWDWT 5 16 January :06 MWT 6 27 January :27 WDWT 7 31 January :23 WDWT 8 4 February :40 MWT 9 5 February :29 NWDWT February :23 SWDWT February :19 NWDWT 12 2 March 1995 and 12:02 SWDWT 3 March :42 SWDWT 13 5 March 1995 and 11:29 WDWT 6 March :19, 12:59 WDWT 14 9 March :27 NWDWT March :39 SWDWT March 1995 and 11:37 MWT 24 March :27 MWT March 1995 and 12:23, 12:12 NWDWT 29 March :12 NWDWT March :02 NWDWT 19 1 April :40 MWT 20 7 April :16 SWDWT April :44 MWT April 1995 and 11:34 NWDWT 12 April :22 NWDWT April :19 WDWT April :23 WDWT April :12 WDWT October :36 WDWT 27 3 November :01, 11:21 NWDWT 28 7 November :18 MWT 29 9 November :56 SWDWT November :07, 12:43 WDWT November :32 WDWT November :23, 12:00 NWDWT November 1995 and 01:32, 11:27 NWDWT 22 November :21, 12:57 NWDWT November :48, 12:24 WDWT 35 1 December :24, 13:00 WDWT 36 4 December :51, 12:27 WDWT 37 6 December 1995 and 12:06 WDWT 7 December :55 WDWT December :54, 12:30 SWDWT December :41, 11:36 SWDWT December :31, 11:26 SWDWT December :29, 13:09 WDWT December :23, 12:58 WDWT frontal band, making the warm front difficult to locate. In this case, the distance between the cold clouds edge and surface front for an active system is typically about 500 km but this will depend, for instance, on the slope of the front.

5 Categorization of weather types using satellite imagery 2955 Table 2. Cumuliform cloud characteristics in satellite imagery according to Rao et al. (1990). Typical grey tone Cloud type Cumulus (Cu) Cumulonimbus (Cb) Stratocumulus (Sc) Typical shape or pattern Small elements or groups of elements Globular or carrot shaped with upwind edge sharp and opposite downwind anvil edge indistinct. Closed cellular pattern in visible imagery over sea in areas of cold advection. Cell size varies depending on inversion height. Visible Medium brightness. Bright. Centre of cells in pattern bright, become grey toward edges where cloud thins. If cells are bellow sensor resolution they appear smooth as if stratus. Infrared Dark tone, usually difficult to detect. Relatively bright. Surface uniform dark grey with cellular structure not present. Can be difficult to detect when cloud tops are low. Table 3. Stratiform cloud characteristics in satellite imagery according to Rao et al. (1990). Typical grey tone Cloud type Typical shape or pattern Visible Infrared Fog-stratus (St) Smooth tops, boundaries often sharp defined by topography. Uniform medium grey tone if thick and mottle tone if thin. Uniform dark grey tone, usually difficult to detect. Altostratus and altocumulus (As and Ac) Smooth tops, boundaries can be ragged or smoothed, often in layers. When in combination with deep convection, tops appear bumpy, if Sun angle low. Cellular structure of altocumulus too small for sensors to resolve so cannot be distinguished from altostratus. Light grey, appears mottled or striated depending on thickness or layered structure. Uniform medium grey tone depending on height. 3. Results Depressional weather types are mainly responsible for cold and rainy weather with strong winds in Greece during the cold period of the year. These weather types are characterized by warm and cold air sequences as well as by potential and kinetic energy exchanges. A typical example is a well-organized depression passing over north Greece. In this case, the warm air initially moves cyclonically towards the Balkan Peninsula and the Black Sea and then, after the intrusion of a polar or

6 2956 C. Cartalis et al. Table 4. Cirriform cloud characteristics in satellite imagery according to Rao et al. (1990). Typical grey tone Cloud type Typical shape or pattern Visible Infrared Cirrus (Ci) Banded fibrous structure. Features of underlying terrain and cumulus clouds are sometimes detectable through clouds. Dark grey to grey tone depending on underlying surface. Light grey in tone. Fibrous structure not as evident as in visible. Cirrostratus (Cs) Generally smooth and uniform tops but sometimes fibrous in appearance. May be in long bands or an extensive sheet. Appears light grey when thin and whiter as thickness increases. Varies from white to grey. Difficult to distinguish from middle clouds. arctic air mass, it comes back as a north-west wind, resulting in a surface pressure increase and in a decrease in surface temperature. Weather conditions depend on evolving air masses, on wind direction at 500 hpa, on the trajectory of the lowpressure system and of the upper trough, on the geographical characteristics of each specific area of interest and on surface roughness. Thirty-six cases for the cold period with a depression passing over the broader Greek area have been examined in this study for the periods January to April and October to December The selection was accomplished by means of cloud system identification in satellite images in combination with the analysis of meansea-level pressure synoptic maps. For each case, the mean depression trajectory was determined by tracking the associated cloud system in successive satellite images. The temporal resolution of the AVHRR images used for the determination of the systems trajectory depended on the number of available images covering the areas close to the position of the systems. The outcome of the above-mentioned analysis was the classification of the depressional weather types in three main categories according to the depression s mean track: West Depressional Weather Type (WDWT), North-west Depressional Weather Type (NWDWT) and South-west Depressional Weather Type (SWDWT). The above-mentioned classification scheme is in accordance with the categorization of depressions made by Maheras (1979, 1983, 1988a). The main trajectories of depression movement over the Mediterranean and southern Europe and the related weather types as they are derived by the analysis of the NOAA/AVHRR images, are illustrated in figure 1. Six cold period cases, which could not be classified as depressional weather types, were also examined. These cases were classified according to Maheras (1979, 1983, 1988a), in a separate category: the Mixed Weather Type (MWT). This weather coincides with severe cold weather, with rainfall and snowfall mainly over central and northern Greece, with very low temperatures and very strong north-easterly winds. In the following sections, each winter weather type is analysed and the most representative case studies of each type are demonstrated. A summary of the cloud characteristics of each weather type in Greece, based on the photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with the characteristics of the systems in synoptic analysis maps, is presented in tables 5 8. Taken that

7 Categorization of weather types using satellite imagery 2957 Figure 1. Main trajectories of depression movement over the Mediterranean and southern Europe and the related weather types as they are derived by analysis of the NOAA/ AVHRR images. no single representative case includes all the observations presented in tables 5 8, some of these observations are not referred to in the text. The frequency of occurrence (in percentage) of each observation associated with each weather type in satellite imagery for the 42 cases is also included in these tables West Depressional Weather Type (WDWT) This category includes all the synoptic situations characterized by the presence of a depression over Greece moving from west to east (the contour lines at the isobaric surface of 500 hpa are parallel to west east direction). Surface temperature variations in these frontal depressions are substantially lower compared to the respective differences in the depressions coming from the north-west because both sectors of the corresponding atmospheric disturbance contain maritime air masses coming from mid-latitudes. In addition, the zonal circulation in the entire troposphere blocks the meridional air mass exchanges, which are responsible for abrupt weather variations. Thus, the zonal momentum and energy advection dominates not only in the surface, but also in the entire troposphere. WDWT may be divided in two sub-categories related to the trajectory of the depression systems: WDWT with north track and WDWT with south track. (a) WDWT with north track: The depression follows a zonal track from west to east at latitude greater than 45 (Maheras 1979, 1983, 1988a). Only the southern

8 2958 C. Cartalis et al. Table 5. Summary of the characteristics of West Depressional Weather Types over Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with synoptic analysis maps. Percentages in satellite observations indicate frequency of occurrence. Synoptic situation 500 hpa analysis (contour lines, temperature, vertical velocity). A trough, frequently located over Italy, with sequential disturbances moving towards NE and E. Cold air masses (with temperatures around 230 C) over Greece. W SW wind currents over Greece. Intensive convective motions over mountainous regions SFC analysis (MSL), 850 hpa analysis (temperature). Depressions associated with cold and warm fronts (or occluded front). The direction of fronts are usually N S. Weak pressure gradients. High levels of relative humidity in specific areas. Weak temperature gradients at 850 hpa. Convective motions over the warm sea in cold sector AVHRR Imagery photointerpretation. Cb embedded in thick Cs and As over the convergence zones of cold and warm air masses (frontal zones of the depressions) 100%. Massive Cs ahead of the cold front of the depressions (75%). Low clouds (St, Sc) over the west side of Pindos mountains due to south-east winds usually ahead of the cold front (100%). Sc orographic clouds over the west side of mountainous areas of Greece due to north-west winds prevailing usually after the cold front of the depression has crossed continental Greece (88%). Few Cs above Ac over the warm sector of the depressions (72%). Low clouds (St, Sc) over the warm sector of the depressions (27%). Streets of Cu and Cb over marine regions when there is cold air advection in the trough of the upper troposphere (50%). Sc or St over sea in front of the warm or occluded front due to the low-level warm advection caused by the south winds (70%) edge of such depression systems affects Greece. The weather is clement, with south winds, variable clouds and very little rainfall. (b) WDWT with south track: This weather type is generated by depressions with Atlantic or Mediterranean origin moving on a zonal track but at a latitude lower than 45. This weather type brings cold weather with rain all over Greece; it is a weather type typical of the winter period (Maheras 1979, 1983, 1988a). In the first case, the depressions with Atlantic origin are attenuated when reaching Greece due to their long path over the Atlantic, west and central Mediterranean; therefore they are associated with occluded fronts. In the second case, the depressions are formed over west or central Mediterranean in association with a south-west extension of the Siberian cold anticyclone, which interrupts the zonal circulation (blocking situation). A summary of the cloud characteristics of WDWT in Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with the characteristics of the systems in synoptic analysis maps, is presented in table 5. In the following sections, two representative case studies of WDWT are demonstrated.

9 Categorization of weather types using satellite imagery 2959 Table 6. Summary of the characteristics of North-west Depressional Weather Types over Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with synoptic analysis maps. Percentages in satellite observations indicate frequency of occurrence. Synoptic situation 500 hpa analysis (contour lines, temperature, vertical velocity). Sequential disturbances moving from NW to SE. Cold air masses (with temperatures around 225 C). Convective motions ahead of the disturbances, especially over the marine regions SFC analysis (MSL), 850 hpa analysis (temperature). Well-developed depressions associated with cold and warm fronts. Low pressure gradients. Weak temperature advection at 850 hpa AVHRR imagery photointerpretation. Cu or Cb over the marine regions behind the cold front as a result of a cold advection and sometimes MCSs and Cb associated with a disturbance in the 500 hpa air flow (80%). Clouds of cold front are composed of either(a) mesoscale convective systems (MCSs) and several Cb (isolated or embedded in thick Cs) (88%) or (b) constitute a typical cold frontal band defined as continuous, relatively broad cloud formation with mostly cold tops (thick Cs and Ci with embedded Cb lowered to Sc at the tail of the front), bright in the visible images, characterized by a distinct long axis with or without curvature (12%). Warm cloud band comprises cloud layers of Ci and Cs and Ac, As underneath and frequently Sc (100%). The direction of high clouds (Ci and Cs) indicates the location of a jet stream which drives the circulation when there is a breakdown of clouds on the right (38%). Sc orographic clouds over the west side of the main mountain range of Greece (Pindos Mountains) and breakdown of clouds over the east side due to north-west winds prevailing, usually after the passage of a cold front (75%). Sc lee waves on the west side of the mountainous Greece and islands in the case of south-west winds over the warm sector of the depression (50%) Case study 1: 5 March 1995 The synoptic situation is presented in figure 2. The 500 hpa height analysis shows that there is a cut-off-low associated with a trough over Italy containing a very cold air mass (230 C). The trough is affecting west Greece while moving towards the east-north-east. The combination of the surface analysis and 850 hpa temperature analysis shows that there is a depression (not very well developed) over the same area associated with a cold front along the western coastline of Greece. Figures 3 and 4 present the AVHRR visible and thermal infrared images, respectively. Both images were recorded on 5 March 1995 at 11:29 UTC. The analysis of consecutive images showed a depression which has just started to develop along a frontal cloud zone north of Greece, moving in a zonal track from west to east. A part of the frontal cloud band associated with the cold front of the depression is evident in the images. It is a classical cold frontal cloud band in the infrared images determined as continuous, relatively broad cloud formation with mostly cold tops, bright in the visible image, and characterized by a distinct long axis with a slight curvature. The position of the cold front has been superimposed in both images. The frontal cloud layer consisting of thick Cs with the overshooting tops of Cb in area A, is clearly depicted in both figures 3 and 4. Cbs are triggered by low-level forcing for ascent along the cold front in combination with considerable forcing associated with an upper level trough. They can be easily recognized from

10 2960 C. Cartalis et al. Table 7. Summary of the characteristics of South-west Depressional Weather Types over Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with synoptic analysis maps. Percentages in satellite observations indicate frequency of occurrence. Synoptic situation 500 hpa analysis (contour lines, temperature, vertical velocity). A trough is located over central or eastern Mediterranean, frequently with sequential disturbances moving from SW to E NE. Intensive upward motions extending from SW and S Greece to west coastline of Turkey. SW winds, frequently very strong SFC analysis (MSL), 850 hpa analysis (temperature). Well-developed depressions located SW of Greece moving towards E NE. The depressions are associated with cold and warm fronts or, frequently, with occluded fronts. High temperature gradients at 850 hpa. High pressure gradients, resulting in very strong winds (frequently stronger than 8 Beaufort over the Aegean) AVHRR imagery photointerpretation. The appearance of Ac at the downwind sides of the mountains (perpendicular to the south winds), indicates the development of lee waves (85%). Massive cloud layers of Sc, As, Ac and Cs are usually observed over the Aegean Sea along a warm front moving to the north (100%). Cb embedded in a thick Cs layer over the cold front, especially when the depressions pass over marine regions (60%). Cu or Cb behind the cold front, especially when the depressions pass over marine regions (100%). Sc formation over the east side of the main mountain range of Greece (Pindos Mountains), in cases in which the depressions are located at the southern Aegean Sea (75%). The appearance of Cs indicates the presence of strong winds in the higher levels of troposphere (location of the jet stream) (60%) the shadow of overshooting tops on the underlying Cs layer in the visible image, as well as from the high brightness values (very low cloud top temperatures) of their tops in the thermal infrared image. There are also Ac and As in the area A, as well as lower clouds as Sc or St. The formation of low clouds is favoured along the west coastline of Greece because the south-easterly winds, which are blown in front of the cold front, transfer warm and wet air masses over this area. The adiabatic cooling of these air masses, as they rise over the Pindos mountains (a mountain chain along the west coastline of Greece), is the main reason for the low clouds formation in the area A. Cs are located in the area B, ahead of the cold front. These clouds are transparent in the visible (figure 3). The areas C and D correspond to thin Ci and Cs, which are characteristic cloud types of the warm section of the depression (Bader et al. 1995). The fibrous nature of Ci can be seen more clearly in figure 3. The very low top temperatures of Ci are obvious in figure 4 (very bright tones) Case study 2: 25 November 1995 The synoptic situation is presented in figure 5. The 500 hpa analysis shows that there is a deep (5580 m) low near Sicily associated with a trough over the Ionian

11 Categorization of weather types using satellite imagery 2961 Table 8. Summary of the characteristics of Mixed Weather Types over Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with synoptic analysis maps. Percentages in satellite observations indicate frequency of occurrence. Synoptic situation 500 hpa analysis (contour lines, temperature, vertical velocity). Low geopotential heights basin over Greece, with perturbations moving from N NW to S SE. Cold air temperatures over Greece (air masses with temperatures around 235 C). Upward motions over marine regions, especially over the location of the depressions. A blocking ridge is situated over west Europe SFC analysis (MSL), 850 hpa analysis temperature). Extended anticyclone covers the Balkan Peninsula, whilst low-pressure systems are located at its south-east edge. The combination of the high-pressure anticyclone with the low-pressure systems, which are located S SE, generates storm N NE winds over the Aegean. Low temperatures at 850 hpa due to the prevailing N winds over the Aegean. The perturbations at 500 hpa enforce the surface winds. The depressions are associated with cold and warm fronts which are moving over the Southern part of Greece AVHRR imagery photointerpretation. Streets of Cu, Cb embedded in As layers and Sc, windward over the Aegean islands, as well as along the east coastline of continental Greece (75%). The appearance of orographic Ac and Sc clouds at the downwind sides of the mountains (perpendicular to the north winds) (50%). Isolated Cu or Cb behind the cold fronts, especially over marine regions (100%). Cu or Cb development over marine regions indicating the locations of 500 hpa perturbations over these areas (100%) Sea. The presence of a depression at the surface in the same area generates strong south winds over the west coastline of Greece. Figures 6 and 7 present the AVHRR visible and thermal infrared images, respectively. Both images were recorded on 25 November 1995 at 12:14 UTC. The exact locations of possible cold and warm fronts are very difficult to determine in this case. In area A, there is a Cs cloud layer above Ac, As and, probably, Sc layers. The stratiform nature of the above-mentioned cloud layers implies that there is synoptic scale air mass convergence without any convective activity in the area A. An inspection of the isotherms (dashed lines at 850 hpa temperature analysis) shows a cold intrusion in the Aegean Sea and a warm advection in the Ionian Sea, consistent with the warm air rising over the cold air in the area A. There is no evidence, however, to indicate that this convergence is associated with an occlusion, warm front or stationary front. The 500 hpa analysis shows that there are descending air mass movements associated with a ridge at mid-tropospheric heights. In area B, there is an extensive opaque cloud layer with very low top temperatures as can be seen in figure 7 (very bright tones). Rough cloud tops in the visible image (figure 6) indicate also convective activity. These vertical motions can be easily explained by the existence of the trough at 500 hpa located over Italy (figure 5(a)) in combination with the low-layer advection of warm air masses (850 hpa temperature analysis, figure 5(c)). This combination is responsible for the high atmospheric instability in area B.

12 2962 C. Cartalis et al. (a) (b) (c) Figure 2. Synoptic analysis for 5 March 1995 (12:00 UTC). (a) 500 hpa height analysis, contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with 5 C intervals). Disturbances in the trough described in the text are indicated with thick dashed lines. (b) 500 hpa vertical velocity analysis with 0.2 Pa s 21 intervals; downward isotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scale map with 4 hpa intervals (solid lines) and 850 hpa temperature analysis (dashed lines with 2 C intervals). Systems described in the text are indicated with thick solid lines.

13 Categorization of weather types using satellite imagery 2963 Figure 3. NOAA/AVHRR channel 1 image (visible) for 5 March 1995 (11:29 UTC). In this case, Greece is affected by a West Depressional Weather Type. The location of the cold front is superimposed in the images. The prevailing cloud systems in areas A, B, C and D are described in the text. In the area C, there is a compact low-level St and Sc layer, probably fog, associated with the cold air intrusion over the Aegean Sea and Greece (figure 5(c)). These low-level clouds or fog are the result of the mixing of this cold air with the warm surface layer over the Mediterranean North-west Depressional Weather Type (NWDWT) This category comprises all the synoptic situations characterized by the presence of a depression over Greece moving on a semi-meridional track from north-west to south-east. This weather type is generated by depressions with Atlantic or Mediterranean origin. Depressions originating from the Atlantic Ocean may reach Greece when the polar front obtains meridional (or semi-meridional) direction over central Europe. Actually, this refers to a secondary branch of the polar front, which is developed when the track of the depressions coming from the Atlantic is oriented in a southeast direction due to the presence of the Atlantic anticyclone. This type of depression is infrequent in Greece, affecting mainly the west Mediterranean or west Europe. Depressions originating from the Mediterranean travel over the central Mediterranean and Italy, moving from north-west to south-east before reaching

14 2964 C. Cartalis et al. Figure 4. NOAA/AVHRR channel 4 image (infrared) for 5 March 1995 (11:29 UTC). In this case, Greece is affected by a West Depressional Weather Type. The location of the cold front is superimposed in the images. The prevailing cloud systems in areas A, B, C and D are described in the text. Greece. In this case, upper level airflow obtains a meridional direction from north or north-west. Mediterranean origin depressions play the most important role for weather and climate in Greece, because they occur frequently and they generate high levels of accumulated precipitation for the whole country. NWDWTs in winter move over Greece along a track situated to the west of the country. If the depressions have Atlantic origin, their frontal systems will reach Greece as occlusions driven by the upper airflow (500 hpa). If the depressions have Mediterranean origin, they will be well developed when they reach Greece. Their impact on weather conditions in Greece is related to the intensity of the cold air intrusion as well as to depth of the associated low-pressure system. This weather type causes cold weather with strong winds and it is also typical of the winter period (Maheras 1979, 1983, 1988a). In the northern part of Greece, this type of depression rarely causes floods, whereas in central and southern Greece, where orographic effects are significant, floods are not unusual. A summary of the cloud characteristics of NWDWTs in Greece, based on the photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with the characteristics of the systems in synoptic analysis maps, is presented in table 6. The case study, which is presented in the following section, has been selected as a representative one.

15 Categorization of weather types using satellite imagery 2965 (a) (b) (c) Figure 5. Synoptic analysis for 25 November 1995 (12:00 UTC). (a) 500 hpa height analysis, contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with 5 C intervals). Disturbances in the trough described in the text are indicated with thick dashed lines. (b) 500 hpa vertical velocity analysis with 0.2 Pa s 21 intervals; downward isotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scale map with 4 hpa intervals (solid lines) and 850 hpa temperature analysis (dashed lines with 2 C intervals). Systems described in the text are indicated with thick solid lines.

16 2966 C. Cartalis et al. Figure 6. NOAA/AVHRR channel 1 image (visible) for 25 November 1995 (12:24 UTC). In this case, Greece is affected by a West Depressional Weather Type. The location of the occluded front is superimposed in the images. The prevailing cloud systems in areas A, B and C are described in the text. Figure 7. NOAA/AVHRR channel 4 image (infrared) for 25 November 1995 (12:24 UTC). In this case, Greece is affected by a West Depressional Weather Type. The location of the occluded front is superimposed in the images. The prevailing cloud systems in areas A, B and C are described in the text.

17 Categorization of weather types using satellite imagery Case study 3: 12 April 1995 The synoptic situation is presented in figure 8. The synoptic analysis shows a depression system moving from north-west to east. The associated cold front is located over north-west Greece, whereas the warm front is located over Thrace (north-eastern Greece). The locations of the above-mentioned fronts have been superimposed to the AVHRR visible and thermal infrared images, which are presented in figures 9 and 10, respectively. Both images were recorded on 12 April 1995 at 11:12 UTC. The stratiform cloud layers in area A (figures 9 and 10) is probably associated with the 500 hpa trough which is oriented in a north-east to south-west direction over Greece (figure 8(a)). There are Cs on top, which obtain very bright tones in visible (figure 9), whereas Ac, As and Sc are located underneath. Ac and As appear as massive opaque clouds in figure 9, whereas Sc obtain grey tones in infrared (figure 10) due to their higher top temperatures. Area B is dominated by a wedgeshaped mesoscale convective system. A narrow cloud band associated with the warm front lies to the north of area B consisting of Cs on top, whereas Ac, As and Sc are located underneath. Frontal clouds in area C are composed of mesoscale convective systems (MCSs) and several Cb, either isolated or embedded in thick Cs, and do not constitute a typical cold frontal band. In satellite images MCSs appear as large circular shields having typical diameters of km, or as elliptical shields with their long axis being up to 1000 km in length (Bader et al. 1995). Few Sc are detected underneath the high cloud tops. Cloud tops in area C of the visible image appear bright with a rugged texture (figure 9) and present relatively low temperatures in the infrared image (figure 10). Altogether are indications of atmospheric instability in this area and therefore of convective activity induced by the upper level forcing in combination with the adiabatic cooling of the corresponding air masses, as they lift over the mountainous west Greece. However, these clouds appear broken in the leeward side of the mountains, due to the downward air motion. Cb and small MCSs behind the cold front indicate convective activity related to the disturbance in the trough at 500 hpa (solid contour lines in figure 8(a)) in combination with the cold advection over the warm sea South-west Depressional Weather Type (SWDWT) This category includes all the synoptic situations characterized by the presence of a depression over Greece following a semi-meriditional track from south-west to north-east. SWDWT may be divided into two sub-categories related to the trajectory of the depression systems: WDWT with West Track and WDWT with East Track. (a) SWDWT with west track: The depression follows a semi-meridional track from south-west to north-east, situated to the west of the Aegean Sea. The synoptic situations related to this category vary according to the location of the low-pressure system. If the warm section of a depression moving slowly towards the north-east affects Greece, the weather will be unstable with moderate cloudiness. If the centre of the depression passes over Greece, a discontinuous middle cloud layer (As) will be developed underneath the high cloud bases. Surface winds are generally southerly, light or moderate. Winds are also southerly at 500 hpa level but much stronger. The descending winds along the west Aegean coastline, which are developed in the leeward main mountain range of

18 2968 C. Cartalis et al. (a) (b) (c) Figure 8. Synoptic analysis for 12 April 1995 (12:00 UTC). (a) 500 hpa height analysis, contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with 5 C intervals). Disturbances in the trough described in the text are indicated with thick dashed lines. (b) 500 hpa vertical velocity analysis with 0.2 Pa s 21 intervals; downward isotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scale map with 3 hpa intervals (solid lines) and 850 hpa temperature analysis (dashed lines with 3 C intervals). Systems described in the text are indicated with thick solid lines.

19 Categorization of weather types using satellite imagery 2969 Figure 9. NOAA/AVHRR channel 1 image (visible) for 12 April 1995 (11:22 UTC). In this case, Greece is affected by a North-west Depressional Weather Type. The locations of the cold and warm fronts of the depressions are superimposed in the images. The prevailing cloud systems in areas A, B and C are described in the text. Greece, contribute to the reduction of relative humidity and of atmospheric instability into the lower atmospheric levels. It has to be mentioned that the cold front associated with the depression reaches Greece from the south. The intrusion of the cold air masses behind the cold front favours the atmospheric instability and the formation of convective clouds over the Aegean Sea. (b) SWDWT with east track: The depression follows a semi-meridional track from south-west to north-east, situated to the east of mainland Greece across the Aegean Sea. This weather type is associated with low temperatures and widespread rainfall with snow in winter (Maheras 1988a). A summary of the cloud characteristics of SWDWT in Greece, is presented in table 7. In the following section a representative case study of SWDWT is demonstrated Case study 4: 18 December 1995 The synoptic situation is presented in figure 11. The surface analysis shows that two low-pressure systems are located north-west and west of Greece, respectively (figure 11(c)). The two surface lows are combined with two troughs at 500 hpa, located in different latitudes (figure 11(a)). Figures 12 and 13 present the AVHRR visible and thermal infrared images, respectively. Both images were recorded on 18 December 1995 at 11:26 UTC. Five specific areas (A, B, C, D and E) are marked in both images.

20 2970 C. Cartalis et al. Figure 10. NOAA/AVHRR channel 4 image (infrared) for 12 April 1995 (11:22 UTC). In this case, Greece is affected by a North-west Depressional Weather Type. The locations of the cold and warm fronts of the depressions are superimposed. The prevailing cloud systems in areas A, B and C are described in the text. The cloud zones in areas A and B are parts of the depression which is crossing southern Greece in a south-west to north-east direction. The areas A and C may be characterized as intense convective zones with Cb clouds embedded into a Cs layer. In the area A, there is a cold front at the surface in conjunction with a disturbance at the 500 hpa level. In the area C, only a disturbance at 500 hpa can be observed. The veering of the geostrophic wind from south-east at surface to south-west at 500-hPa between area A and C indicates warm advection. This appears to reinforce the upper-level forcing associated with the trough. An inspection in both images shows that a Cs layer with stratiform clouds As underneath are located ahead of the areas A and C. In front of the area D, there are isolated Cb that delimit the cold air mass at 500 hpa. These clouds are the results of the thermodynamic instability in the boundary layer caused by the cold air advection over the warmer sea. In the area E, there are mainly St. These clouds present a smooth texture in visible and obtain grey tones in infrared as can be seen in figures 12 and 13, respectively. In the west part of the area E there are also Ac over St. In the area B, there are higher clouds, mainly Ci, related to the warm front, which has been outlined in both images Mixed Weather Type (MWT) The classification of mixed types is made by considering the direction of isobars between an anticyclone and a depression and the wind flow at 500 hpa (Maheras

21 Categorization of weather types using satellite imagery 2971 (a) (b) (c) Figure 11. Synoptic analysis for 18 December 1995 (12:00 UTC). (a) 500 hpa height analysis, contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with 5 C intervals). Disturbances in the trough described in the text are indicated with thick dashed lines. (b) 500 hpa vertical velocity analysis with 0.2 Pa s 21 intervals; downward isotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scale map with 4 hpa intervals (solid lines) and 850 hpa temperature analysis (dashed lines with 2 C intervals). Systems described in the text are indicated with thick solid lines.

22 2972 C. Cartalis et al. Figure 12. NOAA/AVHRR channel 1 image (visible) for 18 December 1995 (11:36 UTC). In this case, Greece is affected by a South-west Depressional Weather Type. The locations of the cold and warm fronts of the depression are superimposed in the images. The prevailing cloud systems in areas A, B, C, D and E are described in the text. 1979, 1983, 1988a). The high-pressure system is located over the broader Greek area, whilst the low is situated at the edge of the anticyclone. The synoptic situation is inverted at 500 hpa level: an extended trough, or a cut-off-low covers the Greek area, whilst a ridge is located at the west endings of the anticyclone. MWT is frequently the result of a jet stream blocking situation in west Europe. The weather conditions frequently depend on the season, similarly to the conditions produced by the depressional weather type. This weather type may be classified in two main sub-categories. The synoptic situation in each sub-category is described as follows (Maheras 1979, 1983, 1988a). (a) In the first case, the anticyclone is located to the north-west or west of Greece and the depression to the north-east or east. Isobars obtain meridional direction at the surface, defining a very strong current of north winds over Greece. (b) In the second case, the anticyclone is positioned to the north-west, north or north-east of Greece, the depression to the south-west, south or south-east. Isobars obtain zonal direction at the surface. A summary of the cloud characteristics of MWTs in Greece, based on photointerpretation of NOAA/AVHRR visible and infrared imagery, in combination with the characteristics of the systems in synoptic analysis maps, is presented in table 8. In the following section a representative case study of MWT is demonstrated.

23 Categorization of weather types using satellite imagery 2973 Figure 13. NOAA/AVHRR channel 4 image (infrared) for 18 December 1995 (11:36 UTC). In this case, Greece is affected by a South-west Depressional Weather Type. The locations of the cold and warm fronts of the depression are superimposed in the images. The prevailing cloud systems in areas A, B, C, D and E are described in the text Case study 5: 23 March 1995 The synoptic situation is presented in figure 14. The surface analysis shows that a low-pressure system associated with a warm and a cold front is located south-east of Greece, whilst an anticyclone is covering the countries of the Balkan Peninsula and the continental part of Greece. The 500 hpa analysis shows that a trough with cold air masses (235 C) is covering the broader Greek area whereas a blocking ridge is situated over west Europe. The curvature of its contour lines indicates high levels of vorticity, which implies high atmospheric instability, especially over the Aegean Sea. Figures 15 and 16 present the AVHRR visible and thermal infrared images, respectively. Both images were recorded on 23 March 1995 at 11:26 UTC. Three specific areas (A, B and C) are marked in both images. In the area A, there is a low-pressure system moving north-eastward. The stratiform cloud layers are obvious in both figures 15 and 16 (Cs on top and underneath Ac, As and probably Sc). Especially in figure 15, the higher clouds are quite massive. This cloud pattern is associated with the warm advection ahead of the warm front. The white arrow in figure 16 shows Cu zones, which determine the location of the cold front. North of area B the north-east winds, which prevail over the Aegean, favour the formation of streets of Cu and Sc as the cold air is warmed flowing over the warm Aegean Sea and the windward side of the islands. The white arrows in both

24 2974 C. Cartalis et al. (a) (b) (c) Figure 14. Synoptic analysis for 23 March 1995 (12:00 UTC). (a) 500 hpa height analysis, contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with 5 C intervals). (b) 500 hpa vertical velocity analysis with 0.2 Pa s 21 intervals; downward isotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scale map with 4 hpa intervals (solid lines) and 850 hpa temperature analysis (dashed lines with 2 C intervals). Systems described in the text are indicated with thick solid lines.

25 Categorization of weather types using satellite imagery 2975 Figure 15. NOAA/AVHRR channel 1 image (visible) for 23 March 1995 (11:37 UTC). In this case, Greece is affected by Mixed Weather Type. The locations of the cold and warm fronts of the depression are superimposed in the images. The prevailing cloud systems in areas A, B and C are described in the text. Figure 16. NOAA/AVHRR channel 4 image (infrared) for 23 March 1995 (11:37 UTC). In this case, Greece is affected by Mixed Weather Type. The locations of the cold and warm fronts of the depression are superimposed in the images. The prevailing cloud systems in areas A, B and C are described in the text.